1. Field of the Invention
The present invention relates to energy technology and more particularly, to a silicon-based photovoltaic cell that, unlike the natural resources of petroleum, natural gas and coal, enhances the photo conversion efficiency of the photovoltaic cell through a red light conversion layer thereon.
2. Description of the Related Art
Photovoltaic cell, more specifically, silicon-based photovoltaic cell is a self-provided energy intensively used in modern technology products such as mobile communication apparatus, microelectromechanical devices and lighting fixtures. For astro navigation, silicon-based photovoltaic cell is the only applicable energy source. Silicon-based photovoltaic cell technology is a special field having concern with the research of solar cell technology.
The design of a silicon-based solar cell started in mid twenty centuries, more specifically, the 1960's decade, when people were enthusiastic about exploring near-earth astro space. US, Soviet and Japanese scientists and engineers are the pioneering promoters of silicon-based photovoltaic cell. These early research results were edited into symposium proceedings (see “Semiconductor Radiation Energy Conversion”, page 408, Moscow, Foreign Literature Publishing Co., Ltd, 1959). According to initial classification, people made the following categories: 1. Photoelectric energy converter; 2. Thermoelectric energy converter, and 3. Semiconductor electromagnetic converter.
FIG. 1 is a schematic drawing showing the structure of a regular silicon-based photovoltaic cell. As illustrated, the silicon-based photovoltaic cell is a device comprising a housing 10, a silicon substrate 20 accommodated in the housing 10, and a p-n junction film 30 on the surface of the silicon substrate 20. The aforesaid silicon-based photovoltaic cell generates energy when radiated by light 40. The aforesaid silicon-based photovoltaic cell further comprises an electrode system 50, a conversion layer 60, and a glass 70 covering the conversion layer 60.
Certain parameters may be applied to explain the characteristics of the silicon-based photovoltaic cell. These parameters include cell voltage V (Voltage), cell current J (Ampere), cell maximum supply power W (Watt), and the most important actual efficiency ζ (%).
As everybody knows, when the light of the sun is radiating the surface of the earth, the radiation energy power on the surface of the earth is 0.1 W/cm2 or 1000 W/m2. With respect to the actual efficiency of silicon-based photovoltaic cell, people estimate the ratio between the power of the cell can reach and the power of the sunlight acted upon the surface of the cell. Many scientific and patent literatures involve the research of the most important characteristic, i.e., the actual efficiency of silicon-based photovoltaic cell. According to huge amount of data computation, for example, use of p-n junctions for solar energy conversion issued by E. S. Rittner (see Phys. Rev. V96N6, 1708, 1954), the efficiency of a material having a wide constraint Eg=1.1 eV is about 25%. This theoretical value is regarded to be very accurate; therefore it shows no significant variation in the last five decades. Based on this theoretical value, a silicon-based photovoltaic cell of surface area 1 m2 provides 250 W electric power under the power of sunlight 1000 W/m2.
Because of high accuracy of the aforesaid theoretical value (see Phys. Rev. V96N6, 1708, 1954), the best silicon-based solar cells developed within the last five decades show the efficiency of 20˜22% (please refer to data from web site www.comp.krit.ru/index.php). Industrial solar cells from many manufacturers around the world, such as “Sun Tech”, “Motech” and etc., show an efficiency about 14.5˜18%. The reason why the actual efficiency is not in conformity with the theoretical computational value is the objective studied in the present invention.
In order to point out the reason why the actual efficiency is not in conformity with the theoretical computational value, the invention compares two curves introduced in Annex I and Annex II. Annex I indicates the radiation of solar spectrum at 38 degrees North Latitude at mid-day in August. With respect to this curve, we made a professional measurement for the sake of excellent reproduction of the value of solar constant 0.1 W/cm2 under these bandwidth and daytime conditions. The characteristics of the solar radiation spectral curve show the type of the extreme-value of the spectrum. The maximum value of the spectrum is at wavelength λ=473±5 nm. This maximum value does not change subject to change of the position and angle of the sun. Only atmospheric phenomena can alter the location of the extreme value, such as thick-fog, claudy and severe raining weather.
Annex II intruduces the nanometered wavelength of the radiation absorbed along transverse axis of coordinates in the silicon-based photovoltaic cell photosensitive spectrum curve, where the axis of coordinates has assigned rating μA/mW. The maximum value of the curve is λ=960˜980 nm, and the photosensitivity is 600 μA/mW. At this time, the total photosensitivity at λ=400 nm is 230 μA/mW. The photosensitivity is higher when at the maximum value area of the solar spectrum. However, this value 320 μA/mW is much lower than the maximum value of silicon photo. The curves shown in Annex I and Annex II are different. Few researches on this matter are made. The applicant of the present invention made a study on this matter and filed an application in Taiwan under Application No. 096105011. This application discloses a compound conversion layer based from two phosphors. This conversion layer is activated by the solar light λmax=470 n, and then gives off light at λ=580˜590 nm. The first phosphor is activated by the wavelength of this luminance. The maximum activation value of the first phosphor is at the spectrum radiation λ=588 nm. The maximum value of the luminesce of the second phosphor is within λ=1000˜1020 nm, i.e., close to the maximum value of the photosensitivity of a silicon-based photovoltaic cell. Based on this prime model, the efficiency of the photovoltaic cell is increased by 25˜30%. Therefore, these silicon-based photovoltaic cells has high efficiency about 16˜19.5%, undoubtly high realistic value.
Although the silicon-based photovoltaic cells and related conversion layers of the aforesaid techniques have a high efficiency value, they still has drawbacks: 1. The use of two phosphors in one conversion layer affects uniformity of the surface illumination of the silicon-based photovoltaic cell; and 2. The use of the second phosphor of radiation wavelength λ=990˜1000 nm causes the silicon-based photovoltaic cell to produce heat.